The invention relates to a method of producing cement from raw material containing harmful substances such as sulphur, alkali, or chlorine.
In rotary kiln production of cement from raw material containing sulphur the sulphur can be present in the following compounds:
CaSO.sub.4.2H.sub.2 O (gypsum) PA1 CaSO.sub.4 (anhydrite) PA1 MgSO.sub.4.2H.sub.2 O PA1 BaSO.sub.4 (barium sulphate) PA1 FeS.sub.2 (pyrite, marcasite) PA1 PbS (lead sulphide) PA1 ZnS (zinc blende) PA1 organic compounds
In all cases the effect of heat causes SO.sub.2 to be produced from these compounds; for example, in accordance with the formula EQU 4FeS.sub.2 +11O.sub.2 =2Fe.sub.2 O.sub.3 +8SO.sub.2
or EQU CaSO.sub.4 =CaO+SO.sub.2 +1/2O.sub.2
Alkali sulphate is formed in the gaseous atmosphere of the rotary kiln from the SO.sub.2 which is present in gaseous form and the vaporized alkali oxide which is also present in the gaseous atmosphere and by absorbing oxygen in accordance with the following: EQU Na.sub.2 O+SO.sub.2 +1/2O.sub.2 =Na.sub.2 SO.sub.4 EQU K.sub.2 O+SO.sub.2 1/2O.sub.2 =K.sub.2 SO.sub.4
If the alkali content in the gaseous atmosphere of the rotary kiln is too low to fix all the SO.sub.2, then calcium sulphate is produced in the hotter zones of the rotary kiln in accordance with: EQU CaO+SO.sub.2 +1/2O.sub.2 =CaSO.sub.4
Finally, if there is a further excess of SO.sub.2 a reaction with the CaCO.sub.3 occurs in the rotary kiln and in the preheater according to the following: EQU CaCO.sub.3 +SO.sub.2 +1/2O.sub.2 =CaSO.sub.4 +CO.sub.2
The alkali sulphates condense in the less hot zones of the rotary kiln and in the preheater on the material to be burnt and return to the sintering zone together with the calcium sulphate formed in the material to be burnt. A small quantity of SO.sub.2 is discharged with the gas leaving the preheater. However, the greater proportion is returned to the sintering zone in the manner described. There most of the sulphur in the form of SO.sub.2 is converted to the gaseous phase and travels with the kiln gas countercurrent to the material. Depending upon the volatility, only a proportion of the sulphur is discharged with the clinker.
Thus, when raw material containing sulphur is used a significant sulphur circulation occurs in the apparatus and this can lead to considerable problems with deposits especially in the inlet region of the rotary kiln. In order to avoid disruption of operation it has been attempted to reduce the sulphur circulation by extracting a proportion of the sulphur-containing gas stream from the apparatus. In the past this was done by causing part of the kiln exhaust gas stream to be branched off after it had left the rotary kiln (i.e., before entering the lowest stage of the cyclone preheater). This procedure also is followed when a specific sulphur content in the clinker must not be exceeded.
Similar problems occur in the production of cement from raw materials containing alkali and/or chlorine. Here too attempts have been made in the past to reduce an undesirable alkali or chlorine circulation by extracting part of the kiln exhaust gas stream (bypass stream) after it had left the rotary kiln.